High-resolution structure of Zn₃(HOTP)₂ (HOTP = hexaoxidotriphenylene), a three-dimensional conductive MOF

Although two-dimensional (2D) electrically conducting metal-organic frameworks (cMOFs) have become prominent due to their numerous potential applications, their structures are often implied or assumed from rather crude powder X-ray diffraction data. Indeed, exceedingly few examples exist of atomic-level structural details coming from single crystal diffraction experiments. Most widely studied among cMOFs are materials based on triphenylene ligands, in particular M₃(HOTP)₂ (M = Cu, Zn) and [M₃(HOTP)₂][M₃(HOTP)]₂ (M = Mg, Ni, Co; H₆HOTP = 2,3,6,7,10,11-hexahydroxytriphenylene), which are invariably described as 2D van der Waals materials with sheets of ligands connected by square planar or octahedral metal ions. Here, we employ electron diffraction to show that, unlike the Mg, Co, Ni, and Cu analogs, Zn₃(HOTP)₂ crystallizes into a three-dimensional network that is analogous to the structures of the lanthanide-based HOTP MOFs. Moreover, similar to the lanthanide frameworks, Zn₃(HOTP)₂ exhibits incommensurate modulation, likely originating from a frustration between the preferred π-π stacking distance and the Zn-O bond lengths, or from a Peierls distortion. This work reinforces the importance of employing single crystal diffraction measurements for the characterization of conductive MOFs, especially when trying to correlate electronic properties to structural details.